The present invention relates to a sheet feeder which is equipped with an image reading apparatus such as a copying machine or a scanner or an image forming apparatus such as a printer or a facsimile machine and which sequentially takes out sheets of document contained in a sheet container in a stacked manner to feed and carry the sheets to a carrier path. The present invention also relates to an image reading apparatus and an image forming apparatus that comprise this sheet feeder.
In recent years, an image forming apparatus or the like has been provided with an apparatus for reading an image of the sheet while automatically and sequentially taking and carrying the sheets of document one by one, so that the image is efficiently read and formed. On the other hand, due to progress of a digital technology, a speed of reading an image from the document, a speed of converting the read data into the electronic data and a speed of forming an image from the electronic data rise, and this makes it possible to process more documents at a high speed. Such a reading apparatus can contain a large number of sheets in a tray (a sheet container), namely, about 100 to 200 sheets. In addition, in accordance with progress of a document carrier of the image reading apparatus, various kinds of sheets can be carried.
Thus, if the number of mounted sheets is increased, in the configuration such that the height position of the tray is fixed, it is difficult to make a taking-out condition of each sheet from the tray even, so that an apparatus has been known, which enables to move up and down the tray by an elevating mechanism and maintains the highest face of the sheet contained n the tray at a predetermined height (see, for example, Japanese Patent Application Laid-Open No. 11-199065 (1999), referred as a prior apparatus below). The prior apparatus intends to acquire an effect of improving a convenience of the containing operation of the sheet by moving down the tray at the lowest position when powering on the apparatus.
In the image reading apparatus for reading the image of the sheet while containing a large number of sheets such as documents in the tray and feeding and carrying them, there is a problem such that it takes a long time from the tray is moved up till the sheet can be fed to a carrier path so as to make a total reading time till reading of the sheet is completed longer when putting a tray having a large capacity for mounting the sheet on standby, moving up the tray by a signal with respect to the key operation or the like for initiating reading of the sheet, and initiating the operation for feeding the sheet to the carrier path for carrying the sheet to a reader.
In the prior apparatus, the tray is moved down to the lowest position even when there are a few sheets to be set, so that it takes a long time for moving up the tray from picking up the sheet till feeding and carrying it. In addition, in recent years, a time from powering on the apparatus till start of the operation has been shortened and an apparatus that is powered off in ordinary state without turning on electricity and is powered on upon using has been developed. Since such an apparatus is turned off usually, before powering on, the sheet may be contained therein. In this case, there is a problem such that the tray is moved down to the lowest position by powering on to cause wasteful operations.
In order to solve such a problem, various methods are tried, for example, before acquiring a signal of reading start, a default height of the tray is set not at the lowest position where the maximum number of sheets can be set but at the higher position capable of setting a predetermined number of sheets; detecting that any key of an operation unit is pushed for setting an operation mode or the like, the tray is moved up so as to stand ready to feed the sheet immediately; and the tray is automatically on standby for feeding the sheet immediately when a predetermined passage of time.
However, according to the apparatus controlled to set the tray not at the lowest position but at a predetermined height, the initial operation of the tray is carried out upon powering on and a standard height of the tray is detected, and then, the tray is moved so as to be at a predetermined height. This is also wasteful operation for making a time from powering on till start of operation shorter as described above.